Heatwaves are a major public health concern causing more fatalities than any other natural hazard. In Australian prisons, extreme temperatures have been recorded and heat related deaths have occurred. While some countries have identified ‘optimum’ thermal conditions for custodial accommodation, there are no national thermal standards, guidelines or recommendations for the design of custodial environments and management of prisoners. This article examines the existing national and international literature to demonstrate that prisoners are a poorly identified subgroup of the population ‘at risk’ of heat-related illnesses. Thermal conditions within prisons and other types of custodial accommodation across Australia vary considerably. This article finds that even if current thermal comfort standards were employed, the prison environments may not be safe or suitable as current methods of devising thermal comfort standards for buildings are flawed as the standards do not take into account the needs of vulnerable users. Given that heatwaves are likely to increase in frequency and duration; devising ‘best practice’ thermal comfort data for the needs of vulnerable users is paramount. Furthermore, the consideration of the thermal design of custodial environments is an important policy, management and design issue that deserves attention in the interests of public and prisoner health both in Australia and internationally.